1. Field of the Invention
The present invention relates to a semiconductor laser control circuit for controlling the laser beam emitted by a semiconductor laser to illuminate an optical recording medium.
2. Related Art Statement
A mark edge recording technique is known as a high-density optical-disk recording technique. In this technique, if a laser is driven by a laser driving current with a simple pulse waveform, accumulation of heat occurs, which causes recorded marks to be distorted into the shape of a teardrop. This distortion causes a problem particularly in the mark edge recording scheme in which data is represented by the length of each recorded mark. That is, the distortion in the mark shape can result in an increase in error during a reproducing operation. One known technique for avoiding the above problem is to drive a laser at a plurality of power levels as shown in FIG. 11. This technique is known as the recording waveform correction.
A semiconductor laser control circuit for achieving the multiple-power-level recording is disclosed for example in Japanese Unexamined Patent Publication No. 2-68736.
In this semiconductor laser control circuit disclosed in Japanese Unexamined Patent Publication No. 2-68736, as shown in FIG. 12, the optical output of a semiconductor laser 101 is detected by a photo detector 102, and the detected signal in current form is converted into voltage form by an operational amplifier 103. The output of the operational amplifier is applied as an optical power control error signal 109 to operational amplifiers 110, 111, and 112 which form a servo system.
In a typical format used in a magneto-optic recording disk, a sector includes a plurality of recording areas (hereafter referred to as MO areas) 114, as shown in FIG. 13(a). Each sector includes preformatted sector marks (SM) 113 located immediately in front of the respective MO areas 114, wherein each sector mark 113 is made for example in the form of a wobble pit by which tracking servo control is performed. Following the first sector mark 113, there are provided a sector address (AD) area 115 and an ALPC (auto laser power control) area 116 wherein the sector address area 115 indicates the sector address and the ALPC area is used to set the optical power levels in reproducing, erasing, and recording operations. FIG. 13(g) illustrates the waveform of the optical output of the semiconductor laser 101 whose output levels are set in the ALPC area.
In FIG. 12, when a reproduction sample gate signal 117 is at a high level, a reproduction sample-and-hold circuit 119 performs a sampling operation. The operational amplifier 110 compares the optical power control error signal 109 with a reproduction reference voltage 118. In accordance with the comparison output of the operational amplifier 110, a reproducing current source 120 is driven, and thus the DC optical-power in the reproducing operation is set to a corresponding level (as represented by reference numeral 121 in FIG. 13(g)). When the reproduction sample gate signal 117 is at a low level, the reproduction sample-and-hold circuit 119 holds the signal received from the operational amplifier 110 so that the DC optical-output power is maintained at the fixed level (as denoted by reference numeral 122 in FIG. 13(g)). The waveform of the reproduction sample gate signal 117 is shown in FIG. 13(d).
In FIG. 12, when a bottom-value sample gate signal 123 is at a high level, a bottom-value sample-and-hold circuit 124 performs a sampling operation. The operational amplifier 111 compares the optical power control error signal 109 with a bottom-value reference voltage 125. In accordance with the comparison output of the operational amplifier 111, a bottom-value current source 126 is driven, and thus the DC bottom-value optical-power (in the erasing operation) is set to a corresponding level (as shown by reference numeral 126 in FIG. 13(g)). When the bottom-value sample gate signal 123 is at a low level, the bottom-value sample-and-hold circuit 124 holds the signal received from the operational amplifier 111 so that the DC bottom-value optical-output power is maintained constant (as denoted by reference numeral 128 in FIG. 13(g)). The waveform of the bottom-value sample gate signal 123 is shown in FIG. 13(e).
In FIG. 12, when a peak-level sample gate signal 129 is at a high level, a peak-value sample-and-hold circuit 130 performs a sampling operation. The operational amplifier 112 compares the optical power control error signal 109 with a peak-value reference voltage 131. In accordance with the comparison output of the operational amplifier 112, a peak-value current source 132 is driven, and thus the DC peak-value optical-power (in the recording operation) is set to a corresponding level (as represented by reference numeral 133 in FIG. 13(g)). When the peak-value sample gate signal 129 is at a low level, the peak-value sample-and-hold circuit 130 holds the signal received from the operational amplifier 112 so that the DC peak-value optical-output power is maintained constant (as denoted by reference numeral 134 in FIG. 13(g)). The waveform of the peak-value sample gate signal 129 is shown in FIG. 13(f).
A modulator stage 135 modulates the optical output power by driving the semiconductor laser 101 with pulse currents having fixed peak and bottom values in accordance with the recording signal 136 as shown in FIG. 13(c). In response to a write gate signal 137, a gate 138 performs gating operation in terms of the recording signal 136. The write gate signal 137 is also used to control the on/off operation of the bottom-value current source switch 139. The waveform of the write gate signal 137 is shown in FIG. 13(b).
In the conventional technique, as described above, it is required that the power level be set by performing test light emission in an ALPC area disposed between a preformatted area and an MO area in a sector as shown in FIG. 13(a) using a circuit such as that shown in FIG. 12. Thus, it is required to set the power for the three different levels in the ALPC area to obtain a corrected recording waveform (write current waveform) such as that shown in FIG. 11.
In the case of a single-hole recording technique in which recording is performed with a single-level optical power, it is possible to set the power in the ALPC.
However, if the circuit shown in FIG. 12 is applied to a high-density recording disk (according to the long-pit Z-CAV recording scheme), the ALPC has only 6 bytes (72 channel bits) in the case of a 130-mm 2.6-GB magneto-optic disk (according to the standard SC23/WG2N 776) as shown in Table 1.
TABLE 1 ______________________________________ Gap Flag Gap ALPC ______________________________________ 5 5 2 6 ______________________________________
In this standard, if the disk rotation speed is selected to 3600 rpm, the channel clock frequency at the outermost zone (33 bands, 1024-byte/sector) of the Z-CAV disk is as high as 67 MHz and the ALPC passing time is as short as 1.075 psec.
To successfully perform the power setting for all the three levels shown in FIG. 11, it is required to complete the power setting for each level in about 300 nsec (.apprxeq.1.075 .mu.sec/3). To achieve .+-.1% accuracy in the above power setting, the semiconductor laser APC (auto power control) loop for each channel in FIG. 12 is required to have a wide frequency bandwidth greater than 17 MHz (17 MHz.apprxeq.(300 nsec/5).sup.-1). This bandwidth is nearly equal to the RF bandwidth of an optical disk recording/reproducing apparatus.
Therefore, it is required that all circuit sections, such as the photo detector for monitoring the semiconductor laser output, current-to-voltage conversion circuit, operating circuit, sample-and-hold circuit, be constructed with components capable of operating in the wide frequency band. Furthermore, three sets of such the wide-band circuit is needed for the respective output levels. This causes the circuit to be complex and expensive.